[mlir] Add notes about using external interface application.

mlir/docs/Interfaces.md

int main() {
 
   // Register the interface model with the type in the given context before
   // using it. The dialect contaiing the type is expected to have been loaded
   // at this point.
   IntegerType::registerInterface<ExternalModelExample>(context);
 }
 ```
 
+Note: It is strongly encouraged to only use this mechanism if you "own" the
+interface being externally applied. This prevents a situation where neither the
+owner of the dialect containing the object nor the owner of the interface are
+aware of an interface implementation, which can lead to duplicate or
+diverging implementations.
+
 #### Dialect Fallback for OpInterface
 
 Some dialects have an open ecosystem and don't register all of the possible
 operations. In such cases it is still possible to provide support for
 implementing an `OpInterface` for these operation. When an operation isn't
 registered or does not provide an implementation for an interface, the query
 will fallback to the dialect itself.